Color-plus-clear coating systems involving the application of the colored or pigmented basecoat to a substrate followed by the application of a transparent or clear topcoat to the basecoat are becoming increasingly popular as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image, and the clear topcoat is particularly important for these properties.
Two-component clearcoat compositions comprising polyisocyanate curing agents and polyols as polyester polyols, polyurethane polyols and acrylic polyols give outstanding gloss and distinctness of image. However, the polyisocyanates are difficult to handle because they are sensitive to moisture and require cumbersome safety precautions because of their toxicity. U.S. Pat. Nos. 4,650,718, 4,681,811, 4,703,101, and 4,764,430 disclose color-plus-clear coatings employing polyepoxides and polyacid curing agents which overcome many problems of polyisocyanate curing agents, but which are still useful as original finishes for automobiles.
A recognized limitation for commercial polyepoxide-based coatings is that commercial embodiments of such coatings are typically two-component systems. Because the polyepoxides and polyacids are typically highly reactive to provide a thorough cure, these components cannot be mixed for a long period of time prior to application. Otherwise, unacceptable increases in viscosity are encountered. Two-component systems require two reservoirs for the different components, as well as separate feed lines and mixing apparatus.
Single-component aminoplast-cured polyol coatings are well-known and provide many excellent properties. However, it is widely recognized that such coatings have poor resistance to etching by acid. Because many geographic areas encounter acid precipitation, these coatings are not highly effective for providing protection for acid resin.
The present invention provides a film-forming composition, which is particularly useful in color-plus-clear coating systems, which has improved stability and improved etch resistance properties. The composition has outstanding gloss and distinctness of image so that the coating is useful as an original finish for automobiles.